Fuel level meter for industrial vehicles

ABSTRACT

The present application discloses a method for measuring pressurized liquid fuel level in a fuel tank used on a vehicle comprising measuring the pressure of the fuel to generate a pressure signal and measuring the temperature of the fuel to generate a temperature signal. A fuel level signal is determined from the pressure signal and the temperature signal and changes in the fuel level signal are determined over time. A first point of the fuel level signal indicating approaching emptying of the tank is identified when changes in the fuel level signal exceed a given value and the approaching emptying of the tank is signaled to an operator of the vehicle in response to identifying the first point. Also disclosed is a fuel level meter system comprising a display comprising a series of at least three light sources arranged in a generally vertical column with a bottommost one of the at least three light sources being of a first color and light sources above the bottommost one of the at least three light sources being of a second color.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/725,531, filed Mar. 17, 2010, and entitled FUEL LEVEL METER FORINDUSTRIAL VEHICLES, which claims the benefit of U.S. Provisional PatentApplication No. 61/161,311, filed on Mar. 18, 2009 and entitled FUELLEVEL METER FOR INDUSTRIAL VEHICLES, the entire disclosures of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to industrial vehicles including forklifttrucks and the like that are operated from liquid fuel contained withinfuel tanks on the vehicles. More particularly, the present inventionrelates to measuring pressurized liquid fuel levels in fuel tanks usedon such vehicles and indicating to operators of the vehicles when thetanks are approaching empty and approximately how much operating timeremains. A common liquid fuel is liquid propane (LP) which is used topower many forklift trucks. Accordingly, the invention will be describedwith reference to forklift trucks which use LP as fuel. It is notedhowever, that the present invention is equally applicable to otherindustrial vehicles that use liquid fuel and liquid fuels other than LPincluding, for example, natural gas (NG). For ease of description,“propane,” “LP” or “fuel” will be used herein to refer to all currentlyavailable liquid fuels and liquid fuels that may become available in thefuture.

Knowing when an energy supply is nearing exhaustion and thecorresponding operating time remaining is important to operators since avehicle may be operated well away from an area where fuel can bereplenished. Typically, industrial vehicles which exhaust their fuelsupplies are difficult if not impossible to move and, if not near or ina refueling area, it may be difficult and time-consuming to refuel them.Accordingly, the remaining fuel and run time is important informationparticularly to newer, less experienced operators.

SUMMARY OF THE INVENTION

The present application discloses a method for measuring pressurizedliquid fuel level in a fuel tank used on a vehicle comprising measuringthe pressure of the fuel to generate a pressure signal and measuring thetemperature of the fuel to generate a temperature signal. A processordetermines a fuel level signal from the pressure signal and thetemperature signal and monitors changes in the fuel level signal overtime. A first point of the fuel level signal indicating approachingemptying of the tank is identified when changes in the fuel level signalexceed a given value and the approaching emptying of the tank issignaled to an operator of the vehicle in response to identifying thefirst point.

The present application also discloses a fuel level meter systemcomprising a display comprising a series of at least three light sourcesarranged in a generally vertical column with a bottommost one of the atleast three light sources being of a first color and light sources abovethe bottommost one of the at least three light sources being of a secondcolor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a lift truck powered by liquid fuel;

FIG. 2 is a block diagram of a fuel system which can use the fuel levelmeter disclosed in the present application;

FIG. 3 is a graph of a measured pressure signal of liquid fuel in aliquid fuel tank, a measured temperature of liquid fuel in a liquid fueltank and a difference signal generated by taking the difference of thetemperature and pressure signals;

FIG. 4 is a graph of a measured pressure signal of liquid fuel in aliquid fuel tank, a measured temperature of liquid fuel in a liquid fueltank, a difference signal generated by taking the difference of thetemperature and pressure signals and a slope of the difference signal;

FIG. 5 illustrates an improved fuel level display for the fuel levelmeter system disclosed in the present application;

FIG. 6 summarizes various display levels showing a composite of adifference signal curve and fuel level displays for the fuel level metersystem disclosed in the present application;

FIG. 7 is flow chart of an embodiment for operation of the fuel levelmeter system disclosed in the present application;

FIGS. 8A and 8B together form a flow chart of an embodiment of a tankchange test for operation of the fuel level meter system disclosed inthe present application;

FIG. 9 is a flow chart of an embodiment of a tank valve check foroperation of the fuel level meter system disclosed in the presentapplication;

FIG. 10 graphically illustrates how the pressure, temperature anddifference signal values change on replacing a first fuel tank with asecond fuller fuel tank; and

FIG. 11 graphically illustrates how the pressure, temperature anddifference signal values change on replacing a first fuel tank with asecond fuller full fuel tank and then on changing the second fuel tankback to the first fuel tank.

DETAILED DESCRIPTION OF THE INVENTION

The temperature and pressure of liquid fuel, commonly liquid propane(LP), inside a fuel tank 100 used on a vehicle, such as a lift truck 102shown in FIG. 1, is monitored in the fuel level meter system of thepresent application. Since the fuel level meter system of the presentapplication is equally applicable to other liquid fuels includingnatural gas (NG), for ease of description, propane, LP or fuel will beused herein to refer to any and all appropriate liquid fuels. Fueltemperature and pressure monitoring can be performed by using a sensor.Conveniently, the sensor 104 (see FIG. 2) is a dual sensing (i.e.,pressure and temperature) sensor, such as the GEMS 3202H200PG028000sensor, that is commercially available from Gems Sensors & Controls,Plainville, Conn.; however, any suitable temperature sensors, pressuresensors and combined temperature and pressure sensors are contemplatedfor use in the fuel level meter system of the present application.

A fuel level signal, generated, for example, by a fuel level meterprocessor 103 by taking the difference between a measured pressuresignal 108 and a measured temperature signal 106 representative of tankconditions, i.e., pressure−temperature (P−T) or temperature−pressure(T−P) is used for fuel level metering (see e.g. FIG. 3). Alternately,the temperature signal and pressure signal can be divided by one anotherto generate the fuel level signal, i.e., P/T or T/P. The processor 103can be a dedicated fuel level meter processor or another processor thatis provided on the lift truck 102. The present application is not to belimited by the fuel system block diagram as shown in FIG. 2 since theposition of components can be moved within the system, for example thefuel filter can be placed before the sensor 104, or components can bemoved and combined with one another, for example the fuel filter couldbe combined with the manifold block.

Due to the properties of gases [Gas law: PV=nRT where P=absolutepressure of gas, V=volume of gas, n=number of moles of gas, R=universalgas constant (189 J/kg K for LP), and T=absolute temperature], if thereis sufficient propane in the tank 100, a normalized temperature signal106 and a normalized pressure signal 108 are highly correlated to oneanother as shown in FIG. 3. In the illustrated embodiment, normalizationis performed by using raw data from analog to digital (A/D) convertersrepresentative of an analog temperature signal and an analog pressuresignal with the normalized values being used to generate the fuel levelsignal. The relationship between the temperature signal 106, thepressure signal 108 and the fuel level signal 110 may be illustrated bygraphing these signals versus time to generate curves representative ofthe signals so that signal and curve may be used interchangeably herein.

In FIG. 3, the illustrated fuel level signal 110 is a difference signal,P−T, which embodiment will be described herein with the understandingthat other relationships, such as T−P, P/T, T/P, can also be used inaccordance with the teachings of the present application. The inventorsof the present application have observed that when graphicallyillustrated, the resulting pressure curve 108 closely resembles thetemperature curve 106 as long as the amount of propane in the tank isgreater than about 9% to 12% of a full tank. While the pressure andtemperature curves 108, 106 may appear jagged, the fuel level signal110, which may be referred to herein as a difference signal ordifference curve, is much smoother than either the pressure curve 108 orthe temperature curve 106 and is relatively flat and fluctuations whichcan occur in the pressure curve 108 due to ambient temperaturevariations are substantially eliminated from the difference curve asillustrated. During the majority of the time the tank is emptying, thefuel level signal 110 does not change substantially, see 110A. It isonly after the fuel in the tank has reached the final 9% to 12% of thefuel that the fuel level signal begins to change, see 110B. At thattime, a point 110C in the difference curve indicating the emptying ofthe final portion of the fuel from the tank can be detected.

During this stage of tank emptying, i.e., below the 9% to 12% fullpoint, changes or reductions in the amount of gas remaining in the tank(changes in n, the number of moles of gas in the gas law equation notedabove) have a noticeable effect on the pressure and temperature signals108, 106. The pressure and temperature signals 108, 106 becomeuncorrelated and the difference curve, the fuel level signal 110, beginsto fall sharply. Because at this time the pressure curve 108 falls andthe temperature curve 106 rises, the fall of the difference curve, thefuel level signal 110, is accentuated.

An LP tank having a 30 pound (13.6 kg) capacity is the industry standardand the 9% to 12% full point for such a standard tank equals about 16minutes of normal running time for the lift truck 102. A variety ofother tank sizes are available, however they are seldom used in theindustry. If tank sizes other than the 30 pound (13.6 kg) capacity sizeare used, the fuel level meter system of the present application stillworks, but the final timing or remaining truck run time would change.That is, a smaller tank would result in less run time after the tankbegins to run out of fuel while a larger tank would result in more runtime after the tank begins to run out of fuel. Also, while normal runtimes for the final portion of fuel will be about the same for vehiclesof a given vehicle model, the normal run times may vary for vehicles ofdifferent vehicle models so that normal run times may need to bedetermined for each vehicle model.

Changes in the difference curve, the fuel level signal 110, such as itsslope and magnitude, are monitored to determine how much fuel remains inthe tank, see the slope curve 112 in FIG. 4 which illustrates the slopeof the difference curve, the fuel level signal 110, versus time. Thereis a first point in the difference curve, the fuel level signal 110,where changes of a predefined amount over time occur as the differencecurve starts to turn down. This first point defines a knee 114 in thedifference curve, the fuel level signal 110. In the illustratedembodiment, the knee 114 is defined as the point at which changes in thedifference curve, the fuel level signal 110, or the slope of thedifference curve reach −11 A/D units in a 20 second time period. Theknee 114 can be detected because it occurs when the pressure andtemperature signals 108, 106 become uncorrelated and the differencecurve, the fuel level signal 110, begins to fall sharply. As notedabove, these changes are due to the reduced amount of fuel in the tank.

In the illustrated embodiment, the difference curve, the fuel levelsignal 110, is sampled every 20 milliseconds with the samples beingaveraged every 2 seconds. The resulting value is compared to the valuedetermined 20 seconds before to determine the change of the fuel levelsignal 110 or slope of the difference curve. It is noted that the kneecan be defined earlier or later, i.e., at a slope of less than −11 A/Dunits/20 sec or at a slope or more than −11 A/D units/20 sec (e.g.between −9 and −11 A/D units/20 sec) or at a slope or more than −11 A/Dunits/20 sec (e.g. between −11 and −13 A/D units/20 sec), depending uponsystem requirements and preferences.

A second point along the difference curve, the fuel level signal 110, isa zero point 116, i.e., the point at which the pressure curve 108crosses the temperature curve 106 in A/D units in the illustratedembodiment. The inventors of the present application have observed thatthe zero point 116 normally is at about the midpoint of the portion ofthe downward sloping difference curve, the fuel level signal 110, thatextends between the knee 114 and the end point or tank empty point 118of the difference curve. The zero point 116 also serves as a referencepoint for predicting how far down the difference curve, the fuel levelsignal 110, will extend beyond the knee 114, i.e., the difference curvegenerally extends as many A/D units beyond the zero point 116 to reachthe tank empty point 118 as the difference curve extends from the knee114 to reach the zero point 116.

Fuel level states exhibited by a fuel level meter or display aredetermined by using the knee 114, the zero point 116 and the tank emptypoint 118 to calculate set points along the downward sloping portion ofthe difference curve, the fuel level signal 110. The set points definetrip locations along the difference curve at which the display isswitched to a higher or lower fuel level indication. In currently usedfuel level displays, two light sources, such as light emitting diodes(LEDs), are used to indicate fuel level. A green LED is illuminateduntil a pressure switch monitoring tank pressure falls below a givenpressure, for example 60 pounds per square inch (PSI). Under normalambient temperatures, a tank pressure of 60 PSI indicates that enoughfuel remains in the tank for about 2 to 3 minutes of truck operation.When tank pressure goes below the given pressure, the green LED isextinguished and a yellow LED is illuminated. Unfortunately, the 2 to 3minutes of remaining operating time can be reduced when the truck isoperating in elevated temperatures and can be increased when the truckis operating in lowered temperatures. Thus, depending on ambienttemperature, the truck runs out of fuel before it is expected or thetruck is refueled before it is necessary to refuel it, possibly wellbefore refueling is necessary, 15 minutes or more.

The fuel level meter of the present application enables an operator tobe alerted more accurately and over a longer period of remainingoperating time to tank empty. While almost infinite alerting schemesusing lights, analog meters, digital meters, LCD displays, and the likecan be imagined, a novel fuel level display 200 is shown in FIG. 2 asbeing connected to the fuel level meter processor 103 and illustrated inmore detail in FIG. 5. The fuel level display 200 has four lightsources, which are conveniently LEDs, although other light sources canbe used. In the illustrated embodiment, the four light sources comprisethree green LEDs 202, 204, 206 and one yellow LED 208. As illustrated,the LEDs are arranged vertically in a column with the yellow LED beingthe bottommost one of the four LEDs. However, it will be appreciatedthat any arrangement of the lights may be used, such as a horizontalarrangement. Similarly, different colors of light may be used, providedan operator of the vehicle can understand what level of fuel is beingsignaled. In the embodiment depicted, prior to reaching the knee 114 ofthe difference curve, the fuel level signal 110, all four LEDs 202-208are illuminated to indicate that sufficient fuel is in the tank 100 topermit truck operation for an extended period of time. As illustrated,this LEVEL 4 display indicates that more than about 16 minutes of truckoperating time remains. Upon reaching the knee 114 of the differencecurve, the fuel level signal 110, the top LED 202 (the top one of thethree green LEDs 202, 204, 206) is extinguished and the remaining threeLEDs 204-208, two green LEDs and one yellow LED, remain illuminated.This LEVEL 3 display indicates that about 16 minutes of truck operatingtime remains.

As noted above, a second point on the difference curve, the fuel levelsignal 110, is defined when the difference curve reaches zero, i.e., thezero point 116. While the second point could be used as the next changefor the fuel level display 200, to provide more increments in thedisplay of run time remaining, four levels are used in the illustratedembodiment. To that end, a third point 120 is defined on the differencecurve 110. The third point 120 is positioned approximately midwaybetween the knee 114, i.e., the first point of the difference curve, andthe midpoint or zero point 116 of the difference curve, i.e., the secondpoint of the difference curve. When the difference curve, the fuel levelsignal 110, reaches the third point 120, the next to the top green LED204 is also extinguished and the two bottom LEDs 206, 208, thebottommost green LED and the yellow LED, remain illuminated. This LEVEL2 display indicates that about 12 minutes of truck operating timeremains.

Upon reaching the second point, i.e., the midpoint or zero point 116, ofthe difference curve, the bottommost green LED 206 is extinguished andthe single bottom LED 208, the yellow LED, remains illuminated. ThisLEVEL 1 display indicates that about 8 minutes of truck operating timeremains. Upon reaching a fourth point 122 positioned between themidpoint or zero point 116 and the tank empty point 118 of thedifference curve, the bottom yellow LED is flashed, an optional audiblealarm is activated and “Out Of Fuel” may be scrolled on a combinationnumerical/text display 210. This LEVEL 0 display indicates that about 4minutes of truck operating time remains. The fourth point 122 isselected to be approximately midway between the second point 120 and theend point 118. Advantageously, the fourth point 122 is selected to beapproximately midway between the second point 120 and the end point 118less a predetermined offset, in order to maintain even spacing in timeof the set points along the difference curve, the fuel level signal 110.In a working embodiment, an offset of 25 A/D units was used so that thefourth point 122 was spaced at the midpoint between the second point 120and the end point 118 −25 A/D units. These levels and displays aresummarized in FIG. 6.

Operation of the illustrated embodiment of the fuel level meter of thepresent application will now be described with reference to a fuel levelmeter flow chart shown in FIG. 7. Upon key-on of the lift truck 102,fuel level meter operation begins at 300. At 302, a determination ofwhether the fuel tank 100 was changed since the truck 102 was lastoperated is made so that an accurate representation of fuel level can beprovided to the operator by operation of the fuel level display 200, see400 of FIG. 8A. The last data stored just before key-off is retrievedfrom nonvolatile memory and if the fuel tank pressure is at least 35 PSIgreater than the last stored fuel tank pressure, it is concluded that afull fuel tank has been installed on the truck 102, the set points alongthe difference curve, the fuel level signal 110, are determined for thefull tank, the fuel level display 200 is set to LEVEL 4 and the tankchange test is ended, see 402, 404, 406, 407.

If tank pressure is not 35 PSI greater than the last stored pressure,temperature and pressure data are taken and averaged for 2 seconds; aninitial fuel level signal 110 (difference signal level value asillustrated) is calculated; and a fuel level signal 110 delta(difference value delta) is calculated by subtracting the ending fuellevel signal 110 (ending difference value retrieved from nonvolatilememory) from the initial fuel level signal 110 (difference value), 408,410, 412.

Pressure and temperature dynamics due to tank changes are illustrated inFIG. 10 for a change to a full tank and FIG. 11 illustrates pressure andtemperature dynamics for a change from an original tank to a full tankand then back to the original tank. If the fuel level signal 110 delta(difference value delta) is greater than ±20 A/D counts of a 10 bit A/Dconverter count, an estimated fuel level signal recovery (differencevalue recovery) is determined and the fuel level signal 110 (differencevalue) is set equal to the initial fuel level signal 110 (initialdifference signal) plus the estimated initial fuel level signal 110recovery (difference value recovery), the resulting fuel level signal110 (resulting difference value), and new set points are determined,416, 418, 420. The estimated fuel level signal recovery will bedetermined empirically and placed into a lookup table that can then beaccessed based on fuel level signal 110 delta (difference value delta).The difference valve recovery accounts for an expected change in thedifference curve as new fuel moves from the tank 100 to the sensor 104.Transit times range from 80 to 100 seconds depending upon fuel demandand a tank change from empty to full could result in a difference curveincrease of up to 70 over that transit period with a 30 pound tank.

If the resulting fuel level signal 110 (resulting difference value) isless than the fourth point 122 (level zero set point), the fuel leveldisplay 200 is set to LEVEL 0, 422, 424. If not, but the resulting fuellevel signal 110 (resulting difference value) is less than the secondpoint 116 (level one set point), the fuel level display 200 is set toLEVEL 1, 426, 428. If not, but the resulting fuel level signal 110(resulting difference value) is less than the third point 120 (level twoset point), the fuel level display 200 is set to LEVEL 2, 430, 432. Ifnot, but the resulting fuel level signal 110 (resulting differencevalue) is less than the first point 114 (level three set point), thefuel level display 200 is set to LEVEL 3. If not, the fuel level displayis set to LEVEL 4, 406.

If the fuel level signal 110 delta (difference value delta) is notwithin ±20 A/D counts of a 10 bit A/D converter count, then it isconcluded that no tank change was made, the previous set points are usedand the tank change test is ended, 438, 440, 407.

After the tank change test sequence is finished, see 407, fuel levelmeter operation continues by averaging the pressure and temperaturesignals for 2 seconds and, in the illustrated embodiment, determiningthe fuel level signal 110 (difference value) by subtracting thetemperature signal in A/D units from the pressure signal in A/D units,304, 306. After a tank change, it is possible that the tank valve willnot be opened. To determine whether that has happened, a check is madefor a tank valve closed condition at 308.

An illustrated embodiment of a tank valve check routine is begun at 500in FIG. 9. A tank valve closed flag, which is false upon entering thetank valve closed routine, is checked to see if the flag has been set totrue at 502. If the tank valve closed flag is false and a timerindicating elapsed time within the tank valve check routine has notexpired, a check is made to see if the pressure in the tank has droppedby greater than 23 PSI over the starting pressure at 506. If so, thetank valve closed flag is set to true and the operator is advised thatthe tank valve is closed, for example by scrolling “Tank Valve Closed”on the combination numerical/text display 210 of the fuel level display200, 508, 510. An optional audible alarm can also be used. The tankvalve check routine is then ended at 512. If the tank valve closed flagis false, the timer indicating elapsed time within the tank valve checkroutine has not expired and the pressure in the tank has not dropped bygreater than 23 PSI over the starting pressure at 506, the tank valvecheck routine is ended at 512.

If the tank valve closed flag is true indicating that the tank valve isclosed, the fuel pressure in the tank is checked to see if it hasrecovered by more than 12 PSI over the starting pressure, 502, 514. Ifthe fuel pressure has not recovered by more than 12 PSI from thestarting pressure, the tank valve check routine is ended at 512. If thefuel pressure has recovered by more than 12 PSI from the startingpressure, the fuel level display 200 is set to LEVEL 2, the optionalaudible alarm is disabled, the “Tank Valve Closed” message is stopped,the tank valve closed flag is set to false and the tank valve checkroutine is ended, 516, 518, 520, 522, 512.

When the tank valve check routine has ended 512 with an indication thatthe valve is closed, the fuel level display 100 is activated to displaya LEVEL 5 alert where all four LEDs 202, 204, 206, 208 are flashed, anaudible alert, if provided, is sounded and “Tank Valve Closed” isscrolled on the combination numerical/text display 210, 310, and fuellevel meter operation returns to 304.

If the tank valve closed flag is false, indicating that the tank valveis open, the fuel level signal 110 (difference value) is used todetermine how to control the fuel level display 200, for example asillustrated at 312 in FIG. 7. The display control illustrated at 312 iscomparable to the display decisions made in FIG. 8 as described above.

Having thus described the invention of the present application in detailand by reference to embodiments thereof, it will be apparent thatmodifications and variations are possible without departing from thescope of the invention defined in the appended claims.

The invention claimed is:
 1. A method for measuring pressurized liquidfuel level in a fuel tank used on an industrial vehicle comprising:measuring the pressure of the fuel to generate a pressure signal;measuring the temperature of the fuel to generate a temperature signal;determining a fuel level signal from said pressure signal and saidtemperature signal by doing one of: dividing the temperature signal bythe pressure signal (T/P) and dividing the pressure signal by thetemperature signal (P/T); detecting changes in said fuel level signalover time; identifying a first point of said fuel level signalindicating approaching emptying of the tank when said pressure signaland said temperature signal become uncorrelated so that changes in saidfuel level signal exceed a given value; and signaling said approachingemptying of the tank to an operator of the vehicle in response toidentifying said first point.
 2. The method of claim 1 furthercomprising: monitoring said fuel level signal after said fuel levelsignal reaches said first point; identifying at least one additionalpoint of said fuel level signal beyond said first point, said at leastone additional point indicating advancement of the approaching emptyingof the tank, said at least one additional point of said fuel levelsignal being approximately half of the fuel level signal at the firstpoint; and wherein signaling the approaching emptying of the tankfurther comprises signaling said at least one additional point to anoperator of the vehicle.
 3. The method of claim 2 further comprisingselecting said at least one additional point of said fuel level signalbeyond said first point to be where said temperature signal and saidpressure signal are substantially equal to one another, and definingsaid point as a second point of the fuel level signal.
 4. The method ofclaim 3 further comprising setting said second point of said fuel levelsignal as a midpoint of a terminal portion of said fuel level signalthat extends between said first point and an end point of said fuellevel signal corresponding to an empty tank.
 5. The method of claim 4further comprising selecting at least one additional point of said fuellevel signal between said first point and said second point indicatingadvancement of the approaching emptying of the tank; and whereinsignaling the approaching emptying of the tank further comprisessignaling said at least one additional point between said first pointand said second point.
 6. The method of claim 5 wherein selecting saidat least one additional point of said fuel level signal between saidfirst point and said second point comprises selecting a third point thatis approximately midway between said first point and said second point.7. The method of claim 6 further comprising selecting at least oneadditional point of said fuel level signal between said second point andsaid end point indicating advancement of the approaching emptying of thetank; and wherein signaling the approaching emptying of the tank furthercomprises signaling said at least one additional point of said fuellevel signal between said second point and said end point.
 8. The methodof claim 7 wherein selecting at least one additional point of said fuellevel signal between said second point and said end point of said fuellevel signal comprises selecting a fourth point that is approximatelymidway between said second point and said end point plus a givenpercentage of a value of said fuel level signal between said secondpoint and said end point of said fuel level signal.
 9. The method ofclaim 2 wherein identifying at least one additional point of said fuellevel signal beyond said first point comprises: identifying threeadditional points of said fuel level signal beyond said first point, thethree additional points indicating progressive advancement of theapproaching emptying of the tank; and wherein signaling the approachingemptying of the tank further comprises signaling the three additionalpoints of said fuel level signal.
 10. The method of claim 1 wherein thepressure signal comprises counts from an analog-to-digital converter andthe temperature signal comprises counts from an analog-to-digitalconverter and subtracting the temperature signal from the pressuresignal comprises subtracting the counts from one another.
 11. The methodof claim 1 wherein said first point corresponds to an amount of liquidfuel remaining in the tank.
 12. A pressurized liquid fuel level metersystem for use on an industrial vehicle comprising: a displaycomprising: a series of at least three light sources arranged in agenerally vertical column; and a bottommost one of said at least threelight sources being of a first color and light sources above saidbottommost one of said at least three light sources being of a secondcolor; and wherein the pressurized liquid fuel level meter systemfurther comprises: at least one sensor for measuring the pressure of thefuel in a fuel tank to generate a pressure signal and for measuring thetemperature of the fuel to generate a temperature signal; a fuel levelmeter processor receiving said pressure and temperature signals anddetermining a fuel level signal therefrom by doing one of: dividing thetemperature signal by the pressure signal (T/P) and dividing thepressure signal by the temperature signal (P/T); said processordetecting changes in said fuel level signal over time; said processoridentifying a first point of said fuel level signal indicatingapproaching emptying of the tank when said pressure signal and saidtemperature signal become uncorrelated so that changes in said fuellevel signal exceed a given value; and said processor signaling saidapproaching emptying of the tank to an operator of the vehicle inresponse to identifying said first point using said display.
 13. Thepressurized liquid fuel level meter system of claim 12 wherein saidseries of at least three light sources comprises four light sources. 14.The pressurized liquid fuel level meter system of claim 12 wherein saidfirst color is yellow and said second color is green.
 15. Thepressurized liquid fuel level meter system of claim 14 wherein saidlight sources are light emitting diodes (LEDs).
 16. The pressurizedliquid fuel level meter system of claim 12 wherein said at least onesensor comprises one dual sensing sensor sensing both temperature andpressure.
 17. The pressurized liquid fuel level meter system of claim 12wherein said first point corresponds to an amount of liquid fuelremaining in the tank.
 18. The pressurized liquid fuel level metersystem of claim 12 wherein a remaining run time of the vehiclecorresponding to the first point of said fuel level signal is dependenton the size of the tank.
 19. A method for measuring pressurized liquidfuel level in a fuel tank used on an industrial vehicle comprising:measuring the pressure of the fuel to generate a pressure signal;measuring the temperature of the fuel to generate a temperature signal;determining a fuel level signal from said pressure signal and saidtemperature signal by doing one of: dividing the temperature signal bythe pressure signal (T/P) and dividing the pressure signal by thetemperature signal (P/T); detecting changes in said fuel level signalover time; identifying a first point of said fuel level signalindicating approaching emptying of the tank when changes in said fuellevel signal exceed a given value over time; and signaling saidapproaching emptying of the tank to an operator of the vehicle inresponse to identifying said first point.
 20. The method of claim 19wherein said first point corresponds to an amount of liquid fuelremaining in the tank.
 21. A pressurized liquid fuel level meter systemfor use on an industrial vehicle comprising: a display comprising: aseries of at least three light sources arranged in a generally verticalcolumn; and a bottommost one of said at least three light sources beingof a first color and light sources above said bottommost one of said atleast three light sources being of a second color; and wherein thepressurized liquid fuel level meter system further comprises: at leastone sensor for measuring the pressure of the fuel in a fuel tank togenerate a pressure signal and for measuring the temperature of the fuelto generate a temperature signal; a fuel level meter processor receivingsaid pressure and temperature signals and determining a fuel levelsignal therefrom by doing one of: dividing the temperature signal by thepressure signal (T/P) and dividing the pressure signal by thetemperature signal (P/T); said processor detecting changes in said fuellevel signal over time; said processor identifying a first point of saidfuel level signal indicating approaching emptying of the tank whenchanges in said fuel level signal exceed a given value over time; andsaid processor signaling said approaching emptying of the tank to anoperator of the vehicle in response to identifying said first pointusing said display.
 22. The pressurized liquid fuel level meter systemof claim 21 wherein said first point corresponds to an amount of liquidfuel remaining in the tank.